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Résumé

This paper considers slotted duty-cycled medium access control
(MAC) protocols, where sensor nodes periodically and synchronously
alternate their operations between active and sleep modes to save
energy. Communications can occur only when nodes are in active
mode. The synchronous feature makes these protocols more
appropriate for delay-sensitive applications than asynchronous
protocols. With asynchronous protocols, additional delay is needed
for the sender to meet the receiver's active period. This is
eliminated with synchronous approaches, where nodes sleep and wake
up all together. Moreover, the contention-based feature makes the
protocols --considered in this paper-- conceptually distributed
and more dynamic compared to TDMA protocols. Duty cycling allows
obtaining significant energy saving vs. full duty cycle
(sleepless) protocols. However, it may result in significant
latency. Forwarding a packet over multiple hops often requires
multiple operational cycles (sleep latency), i.e. nodes have to
wait for the next cycle to forward data at each hop. Timeliness
issues of slotted contention-based MAC protocols are dealt with in
this paper, where a comprehensive review and taxonomy is provided.
The main contribution is to study and classify the protocols from
the delay-efficiency perspective.